Method for triggering retransmission in a multicast system and apparatus implementing the method

ABSTRACT

Disclosed is a method for triggering retransmission in a multicast system, comprising steps of: a base station eNB setting a retransmission condition based on a channel state indication and transmitting the set retransmission condition to a user equipment UE; and the UE comparing the current channel state indication of the UE with the received retransmission condition when receiving an error block, and feeding back a NACK signaling to the eNB trigger a retransmission when the retransmission condition is satisfied. The method according to the present invention is to restrict the trigger of the NACK signaling, by setting the condition of the UE feeding back the NACK signaling when the UE received the error block. Then the feedback signaling on the uplink channel would be restricted, and the retransmission probability would be limited in an appropriate region as in Rel 6 HSDPA, therefore the optimal ratio of efficiency over power would be achieved. Consequently, the number of NACK signaling is restricted so as to avoid the uplink overload. Meanwhile, the retransmission probability is limited so that the optimal ratio of efficiency over power may be achieved.

FIELD OF THE INVENTION

The present invention relates to a multicast system, and especially, toa method for triggering retransmission in a multicast system and anapparatus using the same.

BACKGROUND OF THE INVENTION

E-MBMS is an evolution of multi-media broadcast and multicast service in3GPP LIE. MBMS transmissions may be performed in following two ways:Multi-cell transmissions and Single-cell transmissions. The presentinvention relates to a single-cell transmission mode.

In case of single-cell transmission, i.e. a cell specific to multipointtransmission, a content of multicast service is independent in eachcell. The MBMS data from multiple cells need not synchronously betransmitted. In 3GPP LIE specification, an MTCH (MBMS Traffic Channel)will be mapped to a DL-SCH (Downlink Synchronization Channel).

In E-UTRNA, DL-SCH is mainly mapped by a dedicated traffic channel(DTCH). To improve a peak rate of UE unicast traffic and a systemthroughput, DL-SCH is characterized by using HARQ (High Automatic RepeatRequest), adaptive modulation coding and transmit power, dynamic orsemi-static resource allocation, CQI reporting and etc.

When the MTCH is mapped to the DL-SCH, this part of the DL-SCH should bebroadcasted in the entire cell. To provide a required data rate and BLER(Block Error Ratio) at the edge of cells, similar procedures as used bythe unicast traffic for the DL-SCH are envisaged, such as HARQ.

Similar to HSDPA (High Speed Downlink Packet Access), a physical HARQcan decrease the BLER in a receiver so as to achieve higher transmissionperformance, but a tradeoff is decreasing of transmission efficiency.From the experience of HSDPA deployment, the first transmission's BLERis between 10%˜30%, which could achieve an optimalperformance-efficiency ratio.

However, when the HARQ is applied in multicast systems, since there aremultiple UEs receiving the downlink multicast traffic simultaneously, inprinciple only one UE received an error block and returned a NACKsignaling, a base station (eNB) should retransmit this data block.

From a point view of probability theory, if n UEs are located in onecell, the error probability is α for all the UEs in a firsttransmission, then the probability that at least one UE received theerror block is 1−(1−α)^(n). If the number of the UEs is large, thisprobability would be very high. Therefore some mechanisms should beapplied to restrict the feedback of the NACK signaling, or to triggerthe retransmission of data blocks.

A target is to firstly restrict that the UE whose BLER is less than 30%may feed back the NACK, secondly to limit the probability ofretransmission in eNB, for example 50%. In addition, if the UE feedsback too much NACK, the uplink would be overload. Thus it shouldrestrict the number of NACK signaling upon the UEs receive the errorblock.

The present invention relates to applying HARQ in a method fortriggering a retransmission in a multicast system, which is independentto the way of the UE feeding back the NACK signaling and the way of eNBretransmitting.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, a method for triggeringretransmission in a multicast system is provided, which comprises thefollowing steps of: a base station eNB setting a retransmissioncondition based on a channel state indication, and transmitting the setretransmission condition to a user equipment UE; and the UE comparingthe current channel state indication of the UE with the receivedretransmission condition when receiving an error block. The UE feedsback a NACK signaling to the eNB to trigger a retransmission, when theretransmission condition is satisfied.

According to another aspect of the present invention, a retransmissiontrigger system is provided, comprising:

retransmission condition setting means at a base station eNB, which isadapted to set a retransmission condition based on a channel stateindication and to transmit the set retransmission condition to a userequipment UE; and retransmission comparing means at the UE, which isadapted to compare the current channel state indication of the UE withthe received retransmission condition when the UE receives an errorblock. The UE feeds back a NACK signaling to the eNB to trigger aretransmission when the retransmission condition is satisfied.

The method according to the present invention is to restrict the triggerof NACK signaling, by setting the condition of UE feeding back the NACKsignaling when UE received the error block. Then the feedback signalingon the uplink channel would be restricted, and the retransmissionprobability would be limited in an appropriate region as in Rel 6 HSDPA.Therefore the optimal ratio of efficiency over power would be achieved.Consequently, the number of NACK signaling is restricted so as to avoidthe uplink overload. Moreover, the retransmission probability is limitedso that the optimal ratio of efficiency over power is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of illustrating the present invention withBLER as an example;

FIG. 2 is a schematic diagram of explaining a definition of thresholdaccording to the present invention;

FIG. 3 is a flowchart of a retransmission trigger method according tothe present invention; and

FIG. 4 is a schematic diagram of a retransmission trigger systemaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For E-MBMS, the target is either to extend the cell edge or increase thethroughput. For HARQ, the main point lies in extending the cell edge andfurther providing transmission performances satisfying the BLERrequirement in the cell edge. In FIG. 1, when there is no HARQ, the celledge is a solid curve at BLER of 0.01, where the coverage is 70% asexample. And on the border of coverage with 90%, the BLER is only 0.3.The target of HARQ is to extend the cell edge from 70% to 90% ordecrease the BLER at 90% cell edge from 0.3 to 0.01.

When a retransmission was applied if anyone of UEs reception in the cellis wrong, it would induce 100% retransmission on the assumption that thetens of subscribed UEs were in the cell. The idea retransmissionprobability is less than 30% in Rel 6 HSDPA to achieve the optimal ratioof efficiency over power. The approaches which take the eNB to triggerthe retransmission conditionally will be illuminated in the following.

FIG. 4 is a block diagram of a retransmission trigger apparatusaccording to the present invention. As shown in FIG. 4, a retransmissiontrigger system 400 according to the present invention comprises:feedback condition setting means 401 at an eNB and feedback conditioncomparison means 402 at a UE. The feedback condition setting means 401comprises a static threshold setting module 403, a dynamic thresholdsetting module 405 and an output module 407. The output module 407transmits thresholds generated by the static threshold setting module403 and the dynamic threshold setting module 405 to a reception module408 of the feedback condition comparison means 402 over a controlchannel. The dynamic threshold setting module 405 is optional in thestatic threshold scheme. The feedback condition comparison means 402further comprises a random number generation module 410, a thresholdcomparison module 412, a probability calculation module 414 and a NACKfeedback module 416. When the UE receives an error block, the thresholdcomparison module 412 compares received thresholds with the currentinstantaneous channel state parameter of the UE so as to determinewhether to feed back a NACK signaling to the eNB, thereby facilitatingtriggering a retransmission or not.

In the following, the retransmission trigger method according to thepresent invention will be described in detail by referring to FIGS. 1-4.

Firstly, at step 301, a condition of the UE feeding back the NACKsignaling when the UE receives the error block is set. The condition maybe two thresholds, threshold A and threshold B. The definition of thethresholds may be:

1) Bit Error Ratio (BER) in a physical layer, or2) Block Error Ratio (BLER) in layer 2.

Here the threshold A and threshold B may be configured by the eNB. Asshown in FIG. 2, for example, the actual meanings of threshold A andthreshold B may correspond to the border of coverage in cell.Alternatively, the thresholds may be set using other channel stateindications, such as SNR and the like.

On the assumption that a measured or statistical value is X, X may beBER or BLER in different cases. Then the detailed step is as follows:

when the UE receives an error block, and X is defined as the BER orBLER, whereinif X>A, then UE does not feed back the NACK signaling;if X<B, then UE does not feed back the NACK signaling; andIf X<=A and X>=B, then it is possible for the UE to feed back the NACKsignaling.

Therefore, as shown in FIG. 2, the UEs far from the eNB, which arelocated outside threshold A (e.g. UE3) need not feed back the NACKsignaling. On the assumption that in the identical channel condition,even if there is a retransmission, the probability of correctlyreceiving the previous blocks is very low. The UEs near the eNB, whichare located inside threshold A (e.g. UE1) also need not feedback theNACK signaling since it has satisfied the BLER requirement (less than1%). Only the UEs located between threshold A and threshold B (e.g. UE2)shall feed back the NACK signaling, since HARQ could correct the errorblocks received by UE in this region.

How to choose the threshold and how to implement the method according tothe present invention will be explained in the following with BLER anexample.

On the assumption that there are n UEs in the cell to receive themulticast service, and the instantaneous BLERs are α₁, α₂, . . . ,α_(n), then during one packet transmission, the probability that atleast one UE received error block is:

$P = {1 - {\prod\limits_{i = 1}^{n}\; \left( {1 - \alpha_{i}} \right)}}$

In general, α₁≦α₂≦ . . . α_(n).

If the number of uplinks (slots) that may be simultaneously used in theUE feeding back in RRC CONNECTION, which can be accepted by the eNB, ism, then the following condition shall be satisfied:

${\sum\limits_{i = 1}^{n}\; \alpha_{i}} \leq m$

Therefore, the condition of the UE feeding back the NACK signaling whenUE receives the error block may be set so as to:

1) limit the retransmission probability less than 50%,2) restrict the UE less than 30% feeding back the NACK, and3) restrict the number of simultaneous NACK signaling.

That is, the conditions and thresholds shall be set as:

$\quad\left\{ \begin{matrix}{{1 - {\prod\limits_{i = j}^{k}\; \left( {1 - \alpha_{i}} \right)}} \leq 0.5} \\{{Th\_ A} = {\alpha_{k} \leq 0.3}} \\{{Th\_ B} = {\alpha_{j} \geq 0.05}} \\{{\sum\limits_{i = j}^{k}\; \alpha_{i}} \leq m}\end{matrix} \right.$

wherein j≦i≦k.

At step 302, UEs satisfying the above conditions may feed back NACKsignaling when receiving the error block. Here, we extend the lowerbound from 0.1 to 0.05, and the effective UEs which may feed back theNACK signaling is in this region of 0.05-0.3. The 50% and 30%retransmission probabilities are not the rigorous values, which maydepend on the actual service performance requirement and radio resourceprovided.

Based on the above conditions, two approaches are proposed in thepresent invention to achieve the above targets.

Approach 1: static thresholds, but a dynamic feedback probability factorfor UE receiver is ρ;Approach 2: dynamic thresholds, but the eNB shall know each UE'sinstantaneous BLER and set the thresholds.

As to Approach 1, the eNB sets the static thresholds as Th_A=0.3 andTh_B=0.05, respectively, and transmits them to the UE over a controlchannel. For UE feeding back the NACK signaling, a probability factor ρis set. Once the UE detects the error block, it will feed back the NACKsignaling responding to the HARQ based on the probability factor. Thevalue of the probability factor may be varied related to UE measurementor BLER. For example, if the UE is near the eNB, the measured BLER wouldbe small, then a high probability factor ρ is set by a probabilitycalculation module 414. If the UE is far from the eNB, the measured BLERwould be large, then a low probability factor ρ is set. The detailedstep is as follows:

When the UE receives an error block, and the UE's instantaneous BLER isbigger than Th_A or smaller than Th_B, then the UE does not feed backthe NACK signaling. When the UE's instantaneous BLER is between Th_A andTh_B, a random generation module 410 may generate a random numeral xbetween 0 and 1, wherein:

if x<ρ, then the UE does not feedback an NACK signaling;if x>ρ, then the UE feeds back the NACK signaling by a NACK feedbackmodule 416.

How to Choose the Probability Factor ρ:

Generally, this approach is applied in the scenario where much more UEsexist in a cell. To avoid the overload of the uplink due to thefeedback, even a UE detects an error block, it shall attempt to feedback a NACK signaling respond to the HARQ based on the probabilityfactor. On the assumption that the probability of the UE feeding backthe NACK signaling is ρ_(i), then the probability of the eNB at leastreceiving one NACK is

$P = {1 - {\prod\limits_{i = 1}^{n}\; \left( {1 - {\alpha_{i}\rho_{i}}} \right)}}$

If the number of uplinks (slots) that may be simultaneously used in theUE feeding back in RRC CONNECTION, which can be accepted by the eNB, ism, then the following condition shall be satisfied:

${\sum\limits_{i = 1}^{n}\; {\alpha_{i}\rho_{i}}} = m$

Considering the retransmission probability, P is expected to be theminimal value. Based on the Cauchy inequality, only if α₁ρ₁=α₂ρ₂= . . .=α_(n)ρ_(n), the minimum value can be obtained for P.

By the restriction of ρ_(i)≦1, the feedback probability factor ρ shallbe

$\rho_{i} = \left\{ \begin{matrix}{\frac{m}{\alpha_{i} \cdot n},} & {\alpha_{i} > \frac{m}{n}} \\{1,} & {\alpha_{i} \leq \frac{m}{n}}\end{matrix} \right.$

But there is a problem, that is both the UE and the eNB do not know thetotal number of the UEs in the cell that actually receive the multicastservice, i.e. n.

In order to solve such a problem, a method of an adaptive gradual choiceof ρ value is proposed.

(1) At the beginning of a service transmission, the UE sets an initialvalue κ, for example κ₀=κ_(init)=0.05.(2) Once the i^(th) UE receives κ, it calculates its appropriate ρ_(i)according to the below equality by a probability calculation module 414:

$\rho_{i} = \left\{ \begin{matrix}{\frac{\kappa}{\alpha_{i}},} & {\alpha_{i} > \kappa} \\{1,} & {\alpha_{i} \leq \kappa}\end{matrix} \right.$

wherein α_(i) the current BLER measured by the UE. Therefore, when theUE receives an error block, it generates a random number x between 0 and1, wherein:if x<ρ_(i), then the UE does not feed back the NACK signaling; andif x>ρ_(i), then the UE feeds back the NACK signaling.(3) During a preconfigured duration, the UE takes statistic of the totalretransmission packets, then adjusts the κ:

$\kappa_{j + 1} = \left\{ {{{\begin{matrix}{\lambda\kappa}_{j} & {\frac{d_{error}}{D} < 0.5} \\{1,} & {{{if}\mspace{14mu} {\lambda\kappa}_{j}} > 1}\end{matrix}\kappa_{j + 1}} = {\frac{1}{\lambda} \cdot \kappa_{j}}},\mspace{14mu} {others}} \right.$

where D is the number of packets transmitted in the duration, d_(error)is data of the total retransmission packets in this duration, and λ is aregulation coefficient, λ>1.(4) The UE performs the operation as the step (2).

Another simplest approach for solving the above problem is to adopt afixed ρ, for example ρ=0.3.

Approach 2

A threshold setting module 405 of the eNB sets dynamic thresholds asTh_A and Th_B, respectively, and transmits them to the UE over a controlchannel.

Since the eNB shall know each UE's instantaneous BLER, in this approach,the UE not only feeds back an NACK signaling, but also feeds back theBLER simultaneously in the uplink. The dynamic threshold schemeaccording to the present invention is proposed as:

(1) The eNB sets initial thresholds as Th_A=0.3 and Th_B=0.05,respectively, and transmits them to the UE over the control channel.(2) When the UE receives an error block, and the UE's instantaneous BLERis bigger than Th_A or smaller than Th_B, the UE does not feed back NACKsignaling. When the UE's instantaneous BLER is between Th_A and Th_B,the UE feeds back the NACK signaling in the following format:

UE_id NACK BLER(3) During a preconfigured duration, the eNB takes statistic values α₁,α₂, . . . , α₁ of all the BLER feedbacks from the UE, and then adjustthe Th_A and Th_B based on below equality,

${{\sum\limits_{i = j}^{k}\; \alpha_{i}} \leq m},\mspace{14mu} {\alpha_{j} \leq \alpha_{j + 1} \leq \ldots \leq \alpha_{k}}$${{{If}\mspace{14mu} {\sum\limits_{i = 1}^{l}\; \alpha_{i}}} \geq m},{then}$$\left\{ {{{\begin{matrix}{{{Th\_ A}_{new} = \alpha_{k}},} & {\alpha_{k} = \alpha_{l}} \\{{{Th\_ B}_{new} = \alpha_{j}},} & {{\sum\limits_{i = j}^{l}\; \alpha_{i}} \leq m}\end{matrix}{If}\mspace{14mu} {\sum\limits_{i = 1}^{l}\; \alpha_{i}}} < m},{{then}\left\{ \begin{matrix}{{{Th\_ A}_{new} = {\lambda\alpha}_{k}},} & {\alpha_{k} = \alpha_{l}} \\{{{Th\_ B}_{new} = \alpha_{j}},} & {\alpha_{j} = \alpha_{1}}\end{matrix} \right.}} \right.$

wherein λ is a regulation coefficient, and λ>1;(4) Dynamic threshold could not restrict the probability of the eNB'sretransmission. A retransmission probability factor q in the eNB may beset accordingly, for example q=50%. The eNB may attempt to retransmit anerror transmission block based on the probability factor.

When an eNB receives a NACK signaling, it generates a random number xbetween 0 and 1, wherein:

if x<q, then the eNB retransmits the error transmission block; andif x>q, then the eNB does not retransmit the error transmission block.

The present invention proposes a method for triggering NACK andperforming retransmission in a multicast system for HARQ. Since HARQ isintroduced in the multicast system and the condition retransmissionmechanism is used, higher transmission performance and efficiency may beachieved.

The present invention may restrict the trigger of NACK signaling bysetting the condition of the UE feeding back the NACK signaling when theUE receives the error block. Then the feedback signaling on the uplinkchannel would be restricted, and the retransmission probability would belimited in an appropriate region as in Rel 6 HSDPA, therefore theoptimal ratio of efficiency over power would be achieved. Consequently,the uplink overload may be avoided, since the number of NACK signalingis restricted. Moreover, the optimal ratio of efficiency over power maybe achieved since the retransmission probability is limited.

1. A method for triggering retransmission in a multicast system,comprising steps of: a base station eNB setting a retransmissioncondition based on a channel state indication and transmitting the setretransmission condition to a user equipment UE; and the UE comparingthe current channel state indication of the UE with the receivedretransmission condition when receiving the error block, and feedingback a NACK signaling to the eNB to trigger a retransmission when theretransmission condition is satisfied.
 2. The method according to claim1, wherein the retransmission condition includes a first threshold and asecond threshold higher than the first threshold, and wherein the UEdoes not feed back the NACK signaling to the eNB when the currentchannel state indication of the UE is smaller than or equal to thesecond threshold.
 3. The method according to claim 1, wherein theretransmission condition includes a first threshold and a secondthreshold higher than the first threshold, and wherein when the currentchannel state indication of the UE is between the first threshold andthe second threshold, if the number of UEs that actually receivemulticast services in the cell are known, the UE determines whether tofeed back the NACK signaling or not based on the number of UEs thatactually receive multicast services in a cell, the maximum number ofuplinks simultaneously established that may be used to feed back theNACK signaling, and the current channel state indication of the UE; andif the number of the UEs that actually receive multicast services in thecell are unknown, the UE determines whether to feed back the NACKsignaling based on the total statistical number of retransmissionpackets and the current channel state indication of the UE or not. 4.The method according to claim 3, wherein the UE obtaining a probabilityvalue based on the number of the UEs that actually receive multicastservices in the cell or the statistic total number of retransmissionpackets, the maximum number of uplinks simultaneously established thatare used to feed back the NACK signaling, and the current channel stateindication of the UE so as to generate a random number between 0 and 1,and comparing the generated random number with the probability value,wherein the UE feeds back the NACK signaling to the eNB to trigger aretransmission of the eNB if the random number is bigger than theprobability value, otherwise the UE does not feed back the NACKsignaling.
 5. The method according to claim 1, wherein theretransmission condition includes a first threshold and a secondthreshold higher than the first threshold, and wherein the methodcomprises further steps of: the UE feeding back the NACK signaling tothe eNB to inform the eNB of the current channel state indication of theUE, when the current channel state indication of the UE is between thefirst threshold and the second threshold; the UE updating the firstthreshold and the second threshold based on the received NACK signalingand the channel state indication and transmits the updated thresholds tothe UE; and the UE determining whether to transmit the NACK signalingbased on the updated thresholds or not.
 6. The method according to claim5, wherein the eNB determines whether to retransmit an errortransmission block based on a preset probability factor when receivingthe NACK signaling or not.
 7. The method according to claim 6, whereinthe eNB generates a random number between 0 and 1, and wherein the eNBdoes not retransmit the error transmission block when the presetretransmission probability factor is bigger than the random number,otherwise the eNB retransmits the error transmission block to the UE. 8.The method according to claim 1, wherein the channel state indicationincludes BER, BLER or SNR.
 9. The method according to claim 1, whereinthe eNB transmits the retransmission condition to the UE over thecontrol channel.
 10. A retransmission trigger system, comprising:retransmission condition setting means at a base station eNB, adapted toset a retransmission condition based on a channel state indication andto transmit the set retransmission condition to a user equipment UE; andretransmission comparing means at the UE, adapted to compare the currentchannel state indication of the UE with the received retransmissioncondition when receiving an error block, the UE feeding back a NACKsignaling to the eNB to trigger a retransmission when the retransmissioncondition is satisfied.